The Effect of Electron Heating on Protoplanetary Disk Magnetorotational Turbulence

The Effect of Electron Heating on Magnetorotational Turbulence in Protoplanetary Disks: Self-regulation and Reduced Turbulence Strength

Authors:

Mori et al

Abstract:

The magnetorotational instability (MRI) drives vigorous turbulence in a region of protoplanetary disks where the ionization fraction is sufficiently high. It has recently been shown that the electric field induced by the MRI can heat up electrons and thereby affect the ionization balance in the gas. In particular, in a disk where abundant dust grains are present, the electron heating causes a reduction of the electron abundance, thereby preventing further growth of the MRI. By using the nonlinear Ohm's law that takes into account electron heating, we investigate where in protoplanetary disks this negative feedback between the MRI and ionization chemistry becomes important. We find that the "e-heating zone," the region where the electron heating limits the saturation of the MRI, extends out to 80 AU in the minimum-mass solar nebula with abundant submicron-sized grains. This region is considerably larger than the conventional dead zone whose radial extent is ∼20 AU in the same disk model. Our simple estimate based on the scaling between the Maxwell stress and current density shows that that the MRI turbulence in the e-heating zone should have a significantly low saturation level, with the viscosity parameter α being from 10−5 to 10−3 at the midplane. This implies that the MRI should be "virtually dead" deep inside the e-heating zone. We also find that (sub)micron-sized grains in the e-heating zone are so negatively charged that their collisional growth is unlikely to occur.